Two new classes of papovaviruses of humans, the prototype viruses being BK and JC, are being characterized biologically and biochemically. These viruses are oncogenic for hamsters and transform cells in vitro. They are distant relatives of the well-known SV40, and share virion as well as tumor antigens of the simian virus. The DNA's of all three viruses share some homology, ranging from 10 to 20%. The homology is in the "late" regions of their respective genomes. Although BK grows readily in all human cells tested and therefore can be easily studied by usual virologic methods, the JC virus is extremely fastidious and replicates only in fetal brain cells of human origin. This virus has been most frequently associated with the slow, demyelinating disease of humans, progressive multifocal leukoencephalopathy (PML).